Digital communications systems for the transmission of voice and data are becoming increasingly prevalent. This growth is exemplified by recent Bell System offerings, such as the Bell System's Advanced Communication System and numerous private common carrier offerings of digital service between major cities. An important factor in the use of the various private systems has been the advent of single channel CODEC units, such as the CODEC TP 3000 series. This unit permits digital transmission service to be provided at a bit rate as low as 64 KbS, making possible the use of microwave transmission bands previously available for analog service only.
The increase in digital service at lower bit rates has heightened the need for a digital multiplexer capable of accommodating a large number of analog channels per digital channel. One such multiplexer is shown in U.S. Pat. No. 3,492,932, granted to L. G. Schimpf on Jan. 27, 1970. In Schimpf three incoming analog channels are sampled and encoded into Pulse Code Modulation (PCM) signals by three individual five digit serial encoders. The serial encoder outputs are delayed by predetermined amounts to allow sampling of the time-coincident serial output bits in a predetermined sequence. Each group of successively sampled PCM bits is then converted into a Quantized Pulse Amplitude Modulated (PAM) signal which is representative of a sampled group of PCM bits. The QPAM signal is then transmitted to a receiving terminal where it is decoded into a plurality of respective PCM signals having bit patterns identical to the original encoded analog signals.
Although Schimpf achieves a reduction in intermodulation distortion and saves bandwidth, he requires the use of a separate PAM signal level for each possible combination of the "1" and "0" output bits of the PCM encoders, that is, Schimpf requires an eight level PAM signal to represent three analog channels. If, for example, 30 channels were to be accommodated by Schimpf's method 2.sup.31 or 10 Billion PAM levels would be required. It is apparent that if the number of PAM signal levels could be reduced from the maximum required by Schimpf a marked increase would be achieved in multiplexer capacity.
It is therefore an object of the present invention to improve the capacity of a digital multiplexer.
It is a further object of the present invention to provide a digital multiplexer accommodating the largest possible number of analog channels per digital channel.
It is an additional object of the present invention to transmit a plurality of analog signals over a single digital channel with a minimum amount of bandwidth.
It is a still further object of the invention to increase the number of voice analog channels which can be carried by a digital transmission medium (either cable or microwave) having a limited number of amplitude levels.
Finally, another more specific object of the invention is to transmit an increased number of voice frequency (voice and data) channels over T-1 carrier channels.